The present invention relates to apparatus for measuring the draft of a vessel, comprising a damping chamber for placing in the water in which the vessel is floating. The apparatus includes a gas duct for connection to a manometer, the gas duct having an exit opening below water level in communication with the damping chamber whereby gas can be passed through the gas duct to issue from the gas duct exit opening. The pressure of the gas in the gas duct is representative of the water pressure at the gas duct exit opening. The apparatus also includes a water duct for providing a connection between the damping chamber and the water in which the damping chamber is placed. The water duct has an effective cross-sectional area which is much less than the horizontal cross-sectional area of the damping chamber. The water duct is for admitting water to the damping chamber so that the water level in the damping chamber is above the gas duct exit opening, which water level will, in still conditions, correspond to that of the surrounding water.
The apparatus will be used to enable one to calculate the change in net weight of cargo in a vessel by noting the change in water level, i.e. draft or freeboard, during loading and unloading. At the present time, this is done by observing marks on the hull of the vessel, which is found to be relatively inaccurate; readings which are only slightly wrong can lead to a large miscalculation of cargo weight.
In general terms, it is desirable to provide an apparatus which gives readings which are not excessively influenced even by considerable wave action, which can provide more or less continuous readings at deck level, and which can be designed so as to be easy to handle from deck level.
In the present invention, a closed and vented damping chamber is used, connected to the surrounding water by a water duct having a much smaller effective cross-sectional area than the damping chamber; gas is bubbled from a gas duct terminating in the damping chamber--the pressure of the gas represents the depth of the gas duct exit opening.
British Pat. No. 939,326 discloses apparatus for measuring the draft of a vessel, having the gas duct and the manometer, but not having any damping chamber. In this case, the gas duct issues directly into the water. Although the disclosure recognizes the disturbance that can be caused by waves, the apparatus would be greatly influenced by waves and this would cause inaccurate readings to be taken.
British Pat. No. 189,991, French Pat. No. 466,795 and U.S. Pat. No. 3,396,470 disclose other apparatus which can be used for measuring the draft of vessels.
In general terms, it is desirable to provide an apparatus which gives readings which are not excessively influenced even by considerable wave action, which can provide more or less continuous readings at deck level, and which can be designed so as to be easy to handle from deck level.
Using the invention, the relative cross-sectional areas of the water duct and of the damping chamber enable an average water level to be maintained in the damping chamber, enabling more accurate readings to be taken of the draft or freeboard, from deck level. The damping chamber is suspended at a known distance from deck level and the average depth of immersion is measured by means of the manometer; the damping chamber can be raised or lowered to cross-check readings. Draftmarks on the hull of the ship are not required, and they were often inaccurate. Specifically, the apparatus of the invention compensates for wave action and good readings can be taken even with waves washing over the damping chamber, enabling the apparatus to be used at the quay-side, at anchor or in the open sea. Economically, this is of great importance as vessels can load to the maximum allowed freeboard without risk of overloading. The manometer can be a water manometer containing a sample of the water in which the vessel is floating, thereby compensating the apparatus for water density. As explained in more detail below, false readings due to tidal streams, river currents or water surges can also be reduced or avoided. Rolling of the vessel can be compensated for by averaging the two extremes (maximum level and minimum level).
The apparatus of the invention is easy to handle from the deck by day and night. Although the damping chamber is at sea level, the readings can be taken on deck or even in the ship's office, and it is easy to take cross-check readings and to take readings more or less continuously, as required. In addition, the apparatus can be portable and transported in hand luggage.